Research | Role of Rain in Ocean Mixing & Gas Exchange
Rain impacts on the ocean surface damping the surface waves and generating intense near-surface mixing. We study the influence of rainfall of the generation of turbulence and the subsequent gas flux between the atmosphere and the ocean.
Small-scale ocean surface dynamics, including waves and turbulence, affect global climate through their impact on ocean-atmosphere exchange. However, the role of rainfall on surface fluxes, the dynamics of surface waves, and properties of the marine atmospheric boundary layer have not been studied in detail. Rough estimates indicate that a significant percentage of the shear stress at the ocean surface may be provided, at low wind speed, by rainfall. In addition, rainfall produces significant turbulence in the very near surface layers. This turbulence, in addition to the droplet impacts, disrupts the diffusive molecular surface layers and therefore has the potential to greatly influence air-sea gas fluxes. In this work, laboratory experiments will be performed to directly measure the turbulence generated by rainfall along with the air-water gas, momentum, and heat exchange rates. The role of the dominant parameters such as rain rate, droplet size, wind speed, shear, and turbulence will be measured to quantify the mechanisms by which significant mixing is achieved, and the coupling between the surface fluxes induced by the wind and that induced by the rain will be explored. Secondarily, the dynamic effects of rain-induced stresses on the surface waves will be investigated, specifically with regard to the dependence of the droplet impact angle and velocity (wind speed dependent) on the partition of shear stress (horizontal) and momentum flux (vertical). These effects will further be related to the mechanisms responsible for the surface fluxes. This research is anticipated to lead to a much-needed, improved understanding of the transformation of turbulence within the air boundary layer and fluxes to the upper ocean by rainfall and the consequent enhanced air-sea gas flux.

The figure above shows the turbulent Kinetic energy generated by rainfall on the water surface. One can see both the KE increase as well as the pnetration rate increase with higher rain rates.

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